The California Air Resources Board (CARB) mandates that vehicles powered by an internal combustion engine must include onboard diagnostic systems to monitor the operation of the engine and other components and/or systems related to the operation of the engine to ensure ongoing vehicle compliance with air quality standards.
One of the systems related to the operation of the engine that must be monitored is the engine coolant system. The engine operates most efficiently and produces the least amount of air pollutants when operating above a minimum target temperature. If the vehicle fails to reach the minimum target temperature, then one or more components of the engine coolant system may be malfunctioning or otherwise not operating at an optimum level. Accordingly, CARB mandates that the operation and/or performance of the engine coolant system must be monitored to verify continued proper operation.
In hybrid vehicles, the engine may not operate for a sufficient continuous time to complete a diagnostic check of the engine coolant system. For example, the engine may not operate during auto-stop events and/or engine Deceleration Fuel Cut Off (DFCO) event. Accordingly, the engine must be maintained in an engine running mode during the time required to complete the engine coolant system diagnostic check, thereby inhibiting hybrid vehicles from operating in an engine off mode, such as during an auto-stop event and/or a DFCO event, which reduces the fuel efficiency of the hybrid vehicle.